The invention relates to a high definition color television transmission system in which wide band luminance information is divided to produce a first luminance portion containing the lower frequency components of said luminance information and a second luminance portion containing the higher frequency components of said luminance information, the first luminance portion being transmitted by way of a first transmission path, the second luminance portion being frequency shifted such that the resulting frequency shifted second luminance portion lies within the bandwidth of a second transmission path, which bandwidth is substantially equal to that of the first transmission path, the frequency shifted second luminance portion being transmitted by way of the second transmission path. The invention also relates to television transmission equipment and television reception equipment for use with such a transmission system.
Such a transmission system has been disclosed in the book "Video Tape Recording" by Julian L. Bernstein, published by John F. Rider Publisher, Inc. New York, where in chapter 6, at page 94, it was proposed to divide a television signal into four parts of equal bandwidth and to frequency shift (heterodyne) the three higher frequency parts into the same frequency band as that occupied by the lowest part. The television signal with which this disclosure was concerned was a narrow band signal of 4 MHz bandwidth and was divided and frequency shifted so that it could be recorded on four separate tracks of a tape recorder. It would appear that the television signal in question would be for monochrome display, while from the tape speed, the four tracks would be recorded linearly along the tape. Since the publication of this book, considerable improvements have been made in video recording, though video tape recorders for analog signals still have a restriction on the bandwidth (a few MHz's) that can be recorded.
The color television signal currently transmitted by the broadcast authorities in the United Kingdom uses the PAL system with the transmissions taking place in the U.H.F. bands IV and V. In such transmissions, the luminance information has a bandwidth of 5.5 MHz with a color subcarrier located at a point nominally 4.43 MHz above the carrier frequency, the color subcarrier being modulated by the color information. Substantially all of the currently manufactured color television receivers do not fully utilize all the luminance information transmitted and in fact very little of the luminance information above 3.5 MHz is actually used for the production of the receiver's display. The luminance information above 3.5 MHz is reduced in magnitude to simplify the operation of decoding the color information and with such simplified decoding, cross-color interference would be observed on the display without such reduction. The quality of the display could be improved by using the whole of the luminance information transmitted, but this can only be obtained by a considerable increase in the cost of the receiver's circuitry handling the luminance information and separating it from the modulated color subcarrier.
Various suggestions have been made to further increase the bandwidth and hence the quality and definition, of transmitted color television signals. One such suggestion contemplates the transmission of a single sideband luminance modulated signal of 10 MHz bandwidth with the modulated color subcarrier being located in the other sideband. Such a transmission could not readily be currently transmitted in the U.H.F. bands IV or V as the required bandwidth would overlap into an adjacent channel. In addition, such a transmission could not be received by television receivers currently manufactured and would thus only be receivable by specially constructed receivers. A further suggestion proposed by NHK (Japan Broadcasting Corporation) in their NHK Laboratories Note, Ser. No. 239, August 1979, contemplates the separate transmission of luminance and color information in separate channels and suffers from similar objections to those above.